Devices, such as microfluidic devices have been used for the preparation of a number of compounds, which may be used in medical imaging applications, such as Positron Emission Tomography (PET) systems, that create images based on the distribution of positron-emitting isotopes in the tissue of a patient. The isotopes are typically administered to a patient by injection of probe molecules that comprise a positron-emitting isotope, such as Fluorine-18, covalently attached to a molecule that is readily metabolized or localized in the body or that chemically binds to receptor sites within the body.